1. Field of the Invention
The present invention relates to a logarithmic/inverse-logarithmic conversion circuit which, for example, in radiation measurement or the like, performs logarithmic conversion and inverse-logarithmic conversion for a current signal outputted from a pulse detector for detecting radiation.
2. Description of the Background Art
A detector such as a proportional counter for detecting radiation such as a neutron outputs a pulse signal having a pulse count corresponding to the radiation intensity. The pulse signal is amplified by a pre-amplifier or the like. Then, the amplified pulse signal is converted to a current signal corresponding to the pulse count of the pulse signal by a frequency/current convertor, and the current signal is outputted. As a signal processing circuit for processing such a current signal, a logarithmic/inverse-logarithmic conversion circuit is known which has a combination of an operation amplifier and a transistor and uses a logarithmic characteristic and an inverse-logarithmic characteristic of the transistor (for example, see Patent Document 1).
Here, the reason for providing the logarithmic conversion circuit is that the pulse count outputted from the pulse detector has a wide dynamic range about from 10° counts per second (cps) to 106 cps, and that therefore it is necessary to compress a signal by logarithmic conversion so as to facilitate subsequent signal processing. On the other hand, the reason for the inverse-logarithmic conversion circuit performing inverse-logarithmic conversion for the output signal obtained by logarithmic conversion in the logarithmic conversion circuit is that it is necessary to adjust the gains of a certain range of output signals and closely observe their characteristics.    Patent Document 1: Japanese Laid-Open Utility Model Publication No. 2-63464
The conventional logarithmic/inverse-logarithmic conversion circuit uses a logarithmic/inverse-logarithmic characteristic of a transistor or the like. Therefore, it is necessary to always perform temperature correction in order to keep the linearity of, particularly, the output signal obtained by inverse-logarithmic conversion in the inverse-logarithmic conversion circuit after logarithmic conversion. Thus, a problem of extra time and work for the correction occurs.